In a subsequent pregnancy these antibodies cross the placenta and enter fetal circulation. If the next fetus is also Rh-positive, the mother's antibodies destroy fetal red blood cells.

Before you take the P-Shot to your P (or your V), let's look at purported benefits, how much it may cost, who offers it, and what to expect after it's done.

Research shows exercise can boost your immune system—but it depends on the intensity and consistency of your workouts. Learn how to jumpstart yours.